The present invention relates generally to devices for monitoring the concentration of a fluid.
In semiconductor fabrication, wet cleaning and wet etching are critical steps in the fabrication process. Typically a strong acid such as Hydrofluoric acid (HF), buffered HF (HF & NH.sub.4 F), Sulfuric acid, (H.sub.2 SO.sub.4), Nitric acid (HNO.sub.3) or a combination of different acids are used as cleaning solution. Using an acid having the wrong concentration or the wrong composition will cause significant reduction in the yield of the fabrication process. In order to maintain optimal cleaning, it is important to maintain the concentration level within certain control limits. However, the concentration of the acidic material will often dilute somewhat during use and this results in insufficient cleaning. Accordingly, it is common to frequently replenish the cleaning solution in batch process methods and to maintain a continuous flow of fresh cleaning solution in continuous cleaning methods.
In semiconductor fabrication, wet cleaning and wet etching of the wafers are very critical steps in the fabrication process. One specific cleaning operation is to remove native silicon dioxide (SiO.sub.2) from silicon surface. The silicon dioxide is naturally formed when the silicon is exposed to oxygen in the air. To remove silicon dioxide a cleaning solution that includes a mixture of hydrofluoric acid (HF) and ammonium fluoride (NH.sub.4 F) is used. The mixture ratios typically vary in the range of 1:30 to 1:6 depending on the application. However, it is very important to control the etching rate. Since the process is typically controlled by time, if the etching rate of SiO.sub.2 is too fast, it will cause over etching of base silicon material. Such over etching can alter the characteristics of the electronic components being formed.
Presently, there are no good methods of monitoring the concentration of the solution on a continuous basis during use. Accordingly, a common practice is to periodically clean a test wafer and to measure the actual amount of etching that occurs on the test wafer. The concentration can then be adjusted accordingly. Unfortunately, this is a time and labor consuming process and it also cannot detect any problems that arise between wafer tests. Accordingly, there is a need for a device which can monitor the solution and predict the associated etch rate.
As the manufacturing requirements for large scale integrated circuits become more exacting, new requirements will emerging such as side wall oxide removal. In this process, a weak concentration of acid is used. The weak concentration is used because the material to be etched is so thin that it is crucial to stop etching at a specific point. Accordingly, high concentration solutions are not adequate. However, low concentration solutions tend to deteriorate very quickly (i.e. with only a small amount of etching). Accordingly, current practice utilizes a premixed chemical which is showered over the wafers and drained to the waste tank. However, due to environmental concerns, the costs of such waste disposing is increasing. Therefore, there is a need for a system capable of monitoring solutions having a very weak concentration in an in-line mode without causing any contamination. However, there are no such techniques available currently.
There are two major problems posed by such systems. The first problem is that two different chemicals must be independently detected within an aqueous solution. A single variable detection scheme can not be used to determine the concentration of two different chemicals. The second problem is that existing systems can not provide continuous in-line monitoring without contaminating the solution. In practice, the conventional methods of monitoring concentration levels of a highly corrosive acid solutions are not well suited for monitoring on a continuous basis. The reason is that it is difficult to find contacts and/or electrodes that both work well for their intended function and will not corrode appreciably. In many cases, any corrosive action will be detrimental to both the probes themselves and the cleaning solution. The drawbacks as far as the cleaning solution is concerned is that the dissolved probe material tends to pollute the cleaning solution. The drawback from a probe's standpoint is that their accuracy tends to deteriorate with the corrosion and replacement costs are typically relatively high.
In view of these drawbacks, in most current semiconductor fabrication applications, the concentration of the cleaning solution is not controlled other than to control the concentration of the initial mixture of water and acid. Indeed, the original concentration of the acid (as it is bought from chemical vendors) is not even checked in many cases. Degradation in the cleaning solution and variations in the concentrations are not monitored in any manner during use. Thus, the cleaning solution changing intervals are typically determined on the basis of experience as to what statistically tends to work. In many cases, due to the lack of concentration information, the cleaning solution is prematurely discarded. In a semiconductor fabrication line, the disposal costs of these strong acids account for more than 60% of the total disposal cost and cost savings in this area are becoming more important.
There are currently a variety of practices for measuring acid concentration levels. For example, pH meters can determine ionization to a very accurate resolution. However, the sensor tips are typically made from hygroscopic silica will corrode extensively in strongly acidic solutions such as hydrofluoric acid. Accordingly, they cannot be used for continuously monitoring the cleaning solution. Another common practice is to use a resistivity meter which measures the conductivity of liquid by determining the resistivity between two electrodes that are wetted in the liquid. Resistivity measurement is good for low concentration measurements. However, such measurement are susceptible to temperature and electrodes condition. Selection of a material suitable for use as the electrode is extremely difficult when the solution to be monitored includes a strong acid such as HF. A third common practice is to use a refractometer which measures changes in the refractive index to determine concentration variations in certain aqueous solutions. With hydrofluoric acid, such measurements have the ability to measure concentrations variations on the order of 0.1%. However, again, the prism, which is made of glass or sapphire, is corroded by strong acids such as HF. Accordingly, such monitoring devices are not suitable for continuous monitoring.
Although the described wafer cleaning example is one very good example of an application which requires a non-invasive monitoring of acid concentration levels, it should be appreciated that there are a wide variety of other applications that share the same problems. Accordingly, there is a need to develop an alternative acid concentration detecting apparatus which does not necessarily require contact between the potentially corrosive components of the sensing apparatus and the acid solution.